The purpose of these research projects is to determine whether the mammalian embryo is more sensitive to the tumorigenic and life-shortening effects of X-irradiation than to irradiation after partuition (mouse). Secondly, the importance of dose rate will be evaluated with regard to determining the rate of recovery of irradiated embryos. Thirdly, the extent and persistence of radiation induced biochemical and physiological alterations will be evaluated in the adult animal following their production at specific times during gestation in the rat fetus. Fourthly, studies concerned with variations in the response of the mouse and rat embryo will be continued. Fifthly, the relative importance of irradiation of non-embryonic tissues on embryonic development will be investigated. Attempts have been initiated to irradiate the yolk sac without irradiating the embryo or chorioplacenta, utilizing radioactive labeled antiyolk sac antibodies. Rat zygotes irradiated on the first day of gestation will be studied in order to clarify what factors determine whether the embryo will die or survive. Many of these studies are directed toward determining the threshold level of irradiation for producing many known radiation effects (growth retardation, lethality, CNS defects, etc.) in the embryo and to determine the recuperative powers of the embryo not only as measured at term but also during the post-partum period. The nature of the recovery processes and the factors which influence these recovery processes will be investigated. Finally, the embryo culture technique will be utilized to evaluate the quantitative and qualitative aspects of reproductive hazards, using radiation as the initial agent in order to utilize our extensive in vivo experience.